Patterned magnetic recording media have been proposed to increase the bit density in magnetic recording data storage, such as hard disk drives. In patterned media, the magnetic material is patterned into small isolated blocks or islands such that there is a single magnetic domain in each island or “bit”. The single magnetic domains can be a single grain or consist of a few strongly coupled grains that switch magnetic states in concert as a single magnetic volume. This is in contrast to conventional continuous media wherein a single “bit” may have multiple magnetic domains separated by domain walls. U.S. Pat. No. 5,820,769 is representative of various types of patterned media and their methods of fabrication. A description of magnetic recording systems with patterned media and their associated challenges is presented by R. L. White et al., “Patterned Media: A Viable Route to 50 Gbit/in2 and Up for Magnetic Recording?”, IEEE Transactions on Magnetics, Vol. 33, No. 1, January 1997, 990-995.
Patterned media with perpendicular magnetic anisotropy have the desirable property that the magnetic moments are oriented either into or out of the plane, which represent the two possible magnetization states. It has been reported that these states are thermally stable and that the media show improved signal-to-noise ratio (SNR) compared to continuous (unpatterned) media. However, to achieve patterned media with a bit density of 1 Terabit/in2, a nanostructure array with a period of 25 nm over a full 2.5 inch disk is required. Even though fabrication methods supporting bit densities of up to 300 Gbit/in2 have been demonstrated, large area ultra-high density magnetic patterns with low defect rates and high uniformity are still not available.
The use of multiple level (multilevel) magnetic storage has been proposed, as described in U.S. Pat. No. 5,583,727, but only for continuous (unpatterned) magnetic films and not patterned magnetic islands. However, in multilevel continuous magnetic films the number of magnetic grains, and hence the signal and noise, is divided into the multiple levels, and hence the SNR is degraded.
What is needed is a magnetic recording media and system that takes advantage of both patterned media and multilevel recording.